The use of laser for integrated circuit (IC) marking has become very popular mainly due to its flexibility and speed. In comparison to ink marking, laser marking does not need any dispensable medium such as ink. However, one major disadvantage associated with laser marking is its reduction in contrast. Unlike ink marking where inks are printed onto the IC surface, laser marking operates by removing materials from the IC surface and hence creates a groove-like depression. Materials are being burnt away by the striking laser beam. The roughness of the laser marked surface is different from that of the unmarked surface. The contrast generated by laser marking is a result of these two factors: depression and roughness. Another factor that affects the contrast is the remnant of the burnt compound which has a different reflectivity from the original material.
As far as the external factor is concerned, the apparent contrast of the laser mark is very much dependent on the lighting and the viewing geometry. In other words, the direction of the incident light and the viewing direction determine how well the laser mark can be seen. Usually the depth of a laser mark is about few tens of microns and its roughness is rougher than that of the unmarked surface. In general, one would view the IC marking directly from the top. In this case, the lighting must be set to an oblique angle in order to highlight the contrast of the laser mark. If the lighting is cast from a direction more or less the same as the viewing direction, the laser mark can hardly be seen. The entire IC surface will be brightened up instead.
There have been many attempts to improve the contrast of the laser mark. For example, new molding compound which reacts to laser and produces a contrast between the unmarked and marked surface have been experimented. Another example is the use of different surface glazing material from that of the matrix compound. The laser beam removes the surface material and exposes the underlying matrix compound and hence produces a high contrast mark. However, all these attempts have not been able to fully commercialize due to factors such as cost, etc.